Compression set and compression stress relaxation are both physical properties of materials. These properties are discussed or disclosed for specific materials in elastomeric material data sheets. A measurement of compression set is the percent reduction in dimensional thickness of a material after the material is compressed for a fixed time and at a fixed temperature. A measurement of compression stress relaxation is the reduction in restoration force of a material as the material is being compressed at a constant strain for a fixed time and at a fixed temperature.
Compression set and compression stress relaxation cause negative side effects in platen rollers. For example, a platen roller used in a thermal printer that prints two different media widths experiences negative side effects due to compression set and compression stress relaxation. Specifically, if too many sheets of the smaller width media are printed, especially during an initial utilization, (the initial utilization being approximately the first 1000 prints or the first five percent of the printer life), the rubber in the center portion of the platen roller experiences compression stress relaxation. The rubber in the center portion of the platen roller refers to or corresponds to the location on the platen roller corresponding to the width of the small width media. As a result of the compression stress relaxation, when the larger width media is printed utilizing the platen roller, an image artifact of lighter optical density appears on the larger width media in the location that corresponds to the width of the small width media.
Optical density may be represented in a range from 0 (open air) to 4 (very black). The measurement of optical density is a logarithmic scale where the optical density (OD) value is a negative exponent of the log base 10 value of light transmission (T=10−OD). Light transmission is usually expressed in terms of a percentage, e.g., if OD=0, the light transmission is equal to 100% and all light is being transmitted: if OD=1, light transmission is 10%; if OD=2, light transmission is 1%; if OD=3, light transmission is 0.1%, and if OD=4, light transmission is 0.01%. If one part of a film has a different “background OD” than another part of the film, then the part of the film with the different background OD is considered to have an image quality artifact.
As noted above, this image quality artifact is a degradation of the image quality for the film because the used portion of the roller produces the different optical density on the film when compared to the unused portion. This lighter density is attributed to the reduction in reaction force by the relaxed rubber (in the center portion of the platen roller corresponding to the width of the small width media) which results in less thermal pressure/contact at the media and printbead nip. DIN 6868-56 is a regulatory standard for medical hard copy film imagers and requires that the images produce prints to meet certain image quality guidelines. If the film has a certain number or a certain percentage of image artifacts, the hard copy film imager generating the film may not meet regulatory standard DIN 6868-56, and this makes the medical image printer unusable in a medical imaging environment.
In addition, compression set and compression stress relaxation also cause negative side effects in gasket and O-ring materials. Gasket and O-ring materials tend to leak over time and can no longer provide a sufficient seal. In some cases, this is a result of the gasket or O-ring material relaxing. Initially, when a gasket is tightened, the force that the gasket exerts (pushes) against the mating parts is sufficient to seal properly. This force reduces over time as a result of the compression set and compression stress relaxation property of the gasket and O-ring materials, thus causing the leak.
A prior method to reduce the negative effects of the compression set and the compression stress relaxation is to select a different material for the platen roller, the O-ring, or the gasket. At the molecular level, compression set and compression stress relaxation are the result of the breaking (and subsequent reforming while compressed to try to achieve the lowest energy state) of cross-links in the molecular chains of the compound or material which is utilized to make the roller, O-ring, or gaskets. In order to reduce the breaking of these cross-links, a solution is to utilize a material that is a higher durometer material, i.e., a harder material. In other words, the durometer must be increased. This is a challenge in many applications because, for example, in an application utilizing platen rollers, this would cause small defects in the roller surface to be more likely to show up on the image (if it is a high durometer vs. a low one). In the case of an O-ring, temperature and pressure cycling causes movement of the seal and a high durometer O-ring may not be flexible enough to accommodate this variation and still provide an adequate seal. In other words, a lower durometer material may provide other benefits and these benefits may be essential or beneficial to the functioning of the apparatus that utilizes the roller, O-ring, or gasket. Also, selecting a different material (compound) may result in chemical compatibility issues with other materials being utilized in the apparatus. FIG. 1(a) illustrates restorative force of two materials according to the prior art. As is illustrated in FIG. 1(a), the sample #2 material, which illustratively may be a preferable roller material, has decreasing restorative force which is sloping in a downward fashion even at 70 hours of utilization.
Accordingly, a need exists to have rollers made of a lower durometer material which is not as influenced by compression set and compression stress relaxation in a printing environment where a large format media width and a small format media width are utilized. A need also exists for O-rings, and gaskets to maintain sealing properties and to not leak due to the onset of compression set and compression stress relaxation.